In recent years, the quality of a single crystal according to the CZ method, for example a defect-free crystal, has become higher, and a size thereof has become larger, for example a diameter of 300 mm or more. Particularly in manufacture of the single crystal having a large diameter, when the diameter of the single crystal is too large in comparison with a target diameter due to a detection error of the diameter of the single crystal, it affects finish processing as a cutting loss, and results in a great waste of raw materials.
In a conventional method for detecting the diameter of the single crystal, a meniscus ring, which is seen at the boundary between the crystal and a melt surface in a furnace, is usually captured by using a TV camera fastened at a chamber window. A maximum diameter (a diameter) or a position of the meniscus ring is measured with an image-processing unit from a captured image, and a value of the diameter of the single crystal is calculated on the basis of a placing angle of the camera, a lens to be used, a distance to the crystal and the like.
However, in the event that a relative position of the camera and the crystal changes, for example in the case of a small positional difference of the camera due to re-setting of the chamber after finishing a batch or the case of a positional difference of the melt surface during pulling of the crystal from an expected position, this method generates an error of the calculation of the diameter value.
For example, the melt contained in the crucible decreases, during pulling of the single crystal, by an amount corresponding to the amount of becoming a single crystal, and the position of the melt surface descends. An amount of the descent is accordingly calculated and the crucible is raised with an apparatus for driving the crucible vertically so as to locate the position of the melt surface at its original position. The calculate value, however, may differ from an actual amount of the descent so that the position of the melt surface changes in some cases. As shown in FIG. 7, in the event that the melt surface gradually descends, even when the single crystal has the same diameter, a distance from the camera to the crystal becomes further, and the captured image becomes smaller. As a result, a smaller diameter than an actual diameter is detected so that an error is generated.
The detection error of the diameter of the single crystal due to the position of the melt surface as described above causes an occurrence of variation of the diameter of the manufactured single crystal. That is, when the position of the melt surface is not a desired position, the diameter of the crystal to be grown changes during pulling, and consequently a lower part of the crystal becomes thicker or thinner. There is a difference of a heat environment for a crystal length between the crystal having a thicker lower part and the crystal having a thinner lower part, and the difference causes variation of the quality. In view of this, a method for obtaining the initial position of the melt surface has been suggested to stabilize the diameter of the crystal (See Patent Literature 1, for example).
It is, however, hard to remove the variation of the diameter of the single crystal by this method, and a predetermined diameter is set to a larger diameter to take account of the variation. This causes a problem of decrease in yield.